We have all made the mistake of writing something, only to have it misunderstood by most people (or everyone!) who reads it.
This happens because you know, while you are writing, what it is you want to say, and you put down words that reflect your thoughts. Only lawyers spell out things so precisely and pedantically that no misunderstanding is possible: but this kind of writing is tedious to read – it is dull, boring and repetitive. In real life we abbreviate. Interestingly, there is a branch of linguistics called pragmatics: academics actually study the way in which we say and write things without spelling out every detail, and still understand each other.
The snag with abbreviation is that the reader was not inside your mind when you wrote your instruction manual, or letter, or policy manual. Your reader thus has gaps in understanding: he or she only has the words you wrote. If your words miss out on clearly communicating what was in your thoughts, then the reader is likely to fail to grasp your message.
You would be wise to make use of somebody else to read what you have written and advise you about how to put your point across more effectively.
Make use of an editor!